1. Field of the Invention
The present invention relates to a method of forming a bump. More specifically, the invention relates to a method of forming a bump that has a substantial composition and a well-controlled height.
2. Description of the Related Art
Electronic products containing integrated circuits play an important rolein our daily life. In order to meet the demand for user-friendly electronic products of compactness and multifunction, higher and higher integration of semiconductor devices are necessary and packages thereof are accordingly smaller and smaller. A flip chip package technology has been proposed and widely used to achieve the above-mentioned requirements. In the flip chip package technology, a bump is formed on a bonding pad of a chip. The bump serves to electrically connect the bump to a substrate. Compared to a conventional wire bonding and tape automatic bonding, a circuit path needed for the flip chip package technology is shorter and therefore its electric properties are superior. In some flip chip type packages, a backside of the chip is externally exposed to further improve heat dissipation.
FIGS. 1-7 are schematic views showing a conventional method of forming a bump on a bump pad of a wafer.
In FIG. 1, a wafer 110 having an active surface 112 is provided. A passivation layer 114 and at least one bonding pad 116 exposed by the passivation layer 114 are formed on the active surface 112 of the wafer 110.
In FIG. 2, an adhesive layer 120 is formed over the active surface 112 of the wafer 110 by sputtering to cover the bonding pad 116 and the passivation layer 114. A barrier layer 130 is formed, preferably conformally, on the adhesive layer 120 by sputtering or plating. Then, a wettable layer 140 is formed on the barrier layer 130 by sputtering or plating. An under ball metallurgy (UBM) consisting of the adhesive layer 120, the barrier layer 130 and the wettable layer 140 is thus accomplished.
In FIG. 3, the UBM is subject to a photolithography process. First, a photoresist 150 is formed on the wettable layer 140, and defined by exposure/development to form least an opening 152 in the photoresist 150. The opening 152 exposes a portion of the wettable layer 140 above the bonding pad 116.
In FIG. 4, a conductive material is filled into the opening 152 to form a conductive stud 160. The conductive stud 160 covers the exposed portion of the wettable layer 140.
In FIG. 5, the photoresist 150 is removed by a conventional removing process until the UBM 142 beneath the photoresist 150 is exposed. The exposed UBM 142 is subsequently removed by etching to expose the passivation layer 114 thereunder, as shown in FIG. 6. Only the portion of the UBM 142 beneath the conductive stud 160 exists.
In FIG. 7, a reflow process is performed to melt and reform the conductive stud 160 into a ball-shaped bump 170. The bump 170 consists of the UBM 142 and conductive stud 160.
In FIG. 1 to FIG. 7, the conductive stud 160 is formed in the opening 152 of the photoresist by plating. The composition ratio of the conductive stud 160 formed by plating can not be precisely controlled, especially in the case of forming the conductive stud with high lead or no lead content. In the case of forming the leadless conductive stud, tin silver alloy, tin silver copper alloy or tin silver bismuth alloy is used as the material to form the leadless conductive stud. However, it is difficult to control a plating liquid with a constant composition.